1. Field of the Invention
This invention relates to a process for removing H.sub.2 S from a gas using NMP. More particularly, this invention relates to an improved process for scrubbing H.sub.2 S from a gas with NMP to form an H.sub.2 S-rich solution which is then heated and sent to a stripping zone to remove the H.sub.2 S from the NMP wherein the improvement comprises using hot NMP recovered from extracted hydrocarbon oil to heat the H.sub.2 S-rich NMP solution before same is sent to said stripping zone. Still more particularly, in a process for removing H.sub.2 S from a hydrofiner tail gas wherein said H.sub.2 S-containing gas is passed into a scrubbing zone wherein it is contacted with liquid NMP to remove most of the H.sub.2 S from the gas to form an H.sub.2 S-rich NMP solution, heating said H.sub.2 S-rich NMP solution and passing said hot solution to a stripping zone to remove most of the H.sub.2 S from the NMP to form an H.sub.2 S-lean NMP solution extracting a hydrocarbon oil with said H.sub.2 S-lean NMP solution and recovering hot NMP from said hot extracted oil, the improvement which comprises heating said H.sub.2 S-rich NMP solution to the temperature required for stripping same by indirectly contacting same, in heat exchange relationship, with at least a portion of said hot, liquid NMP recovered from said extracted oil.
2. Description of the Prior Art
It is known to use NMP to scrub or absorb H.sub.2 S from various H.sub.2 S-containing gas streams thereby forming an H.sub.2 S-rich NMP solution. It is known to regenerate the NMP by heating up the H.sub.2 S-rich NMP solution and stripping off the H.sub.2 S in a stripping zone, cooling the so-regenerated NMP and recycling it back into the scrubber. It is also known in the art to use NMP for hydrocarbon extraction processes such as the BTX process for separating paraffins from gasoline fractions and for lube oil extraction processes. In all of the hydrocarbon extraction processes employing NMP as the extraction solvent, it is economically necessary to recover the solvent. This is generally done by a combination of flash evaporation and distillation, with gas stripping used to remove residual amounts of NMP from the extracted hydrocarbon. The solvent-free hydrocarbon is then often sent to a hydrofiner for final purification by contact with hydrogen and a catalyst wherein polar compounds containing sulfur, nitrogen and oxygen, as well as color bodies, and unstable compounds, etc., are removed. The gas exiting from a hydrofiner is generally relatively rich in hydrogen. By way of illustrative, but non-limiting example, a typical tail gas from a lube oil hydrofiner will comprise about 71% mole hydrogen, 13% methane, 5% ethane, 5% C.sub.3.sup.+ alkanes and 6% H.sub.2 S. Thus, it can be seen that this gas contains considerable quantities of H.sub.2 S. This gas may be used again in another hydrofiner in order to consume more of the hydrogen or it may be sent directly to a scrubber in another part of the refinery to remove the H.sub.2 S by scrubbing with an amine such as MEA. The H.sub.2 S is then sent to a sulfur recovery plant and the scrubbed gas can be used as fuel or for other purposes.
It would be advantageous to use this H.sub.2 S-containing tail gas for gas stripping in a lube oil or other extraction solvent recovery system on a once-through basis and ultimately use the spent gas from the solvent recovery system as fuel in the solvent recovery furnaces. However, in order to accomplish this, the H.sub.2 S must be removed from the gas prior to its use in the solvent stripping operation in order to avoid excessive metal corrosion in the solvent recovery system.
It is undesirable to scrub the gas with caustic soda solution, because the high levels of hydrogen sulfide in the gas would require excessive quantities of caustic and also because of the pollution problem caused by the disposal of large amounts of spent caustic. Scrubbing the gas with MEA, DEA or Catacarb is feasible, but this would introduce a whole new system of scrubbing and regeneration, pumps, solution storage, filters, etc., as well as introducing potential NMP solvent contamination to the H.sub.2 S extraction process. Therefore, it would be advantageous if a way could be found to strip hydrogen sulfide from these gases in an economical manner and in a way such that the H.sub.2 S gas scrubbing and NMP regeneration process is integrated with the solvent recovery operation or section of an NMP solvent extraction process, particularly if a way could be found to do this without having to introduce any additional energy or contaminants in the system which is required as taught by the prior art.